1. Get the trouble code(s) read since the D4 light is flashing. Many places like Autozone, etc. will read the code(s) for no charge. Then post the trouble code number(s).

2. Was the transmission cooler and cooler lines flushed prior to installing the replacement transmission. The transmission cooler is part of the radiator.

If the cooler and lines were not flushed, contaminants may cause issues with the replacement transmission.

3. Were the solenoids re-used on the replacement transmissions?

See this excerpt from a Honda Service News article.